It is often necessary or desirable to determine the location of a medical probe relative to a location of interest within three-dimensional space. In many procedures, such as interventional cardiac electrophysiology therapy, it is important for the physician to know the location of a probe, such as a catheter, (especially, a therapeutic catheter) relative to the patient's internal anatomy. During these procedures, a physician, e.g., steers an electrophysiology mapping catheter through a main vein or artery into the interior region of the heart that is to be treated. The physician then determines the source of the cardiac rhythm disturbance (i.e., the targeted cardiac tissue) by placing mapping elements carried by the catheter into contact with the heart tissue, and operating the mapping catheter to generate an electrophysiology map of the interior region of the heart. Having identified the targeted cardiac tissue, the physician then steers an ablation catheter (which may or may not be the same catheter as the mapping catheter above) into the heart and places an ablating element carried by the catheter tip near the targeted cardiac tissue, and directs energy from the ablating element to ablate the tissue and form a lesion, thereby treating the cardiac disturbance.
In certain advanced electrophysiology procedures, it is desirable to create a lesions around, within, or otherwise adjacent to orifices. For example, as part of the treatment for certain categories of atrial fibrillation, it may be desirable to create a curvilinear lesion around or within the ostia of the pulmonary veins (PVs), and a linear lesion connecting one or more of the PVs to the mitral valve annulus. To do this, a physician must be able to move the ablation catheter tip along a desired path and either deliver ablative energy while slowly dragging the tip along the path, or deliver energy at a number of discrete points along that path. Either way, it is crucial that the physician be able to accurately and controllably move the catheter tip along that path by referencing anatomical structures, such as the ostium of the PV's and the mitral valve annulus. More importantly, during the electrophysiology procedure, it is important to prevent inadvertent damage to non-targeted regions, such as the PVs themselves, which could produce stenosis of the PVs.
Traditionally, navigation of catheters relative to points of interest has been accomplished using fluoroscopy. In this case, radiopaque elements are located on the distal end of the catheter and fluoroscopically imaged as the catheter is routed through the body. As a result, a two-dimensional image of the catheter, as represented by the illuminated radiopaque elements, is generated, thereby allowing the physician to roughly determine the location of the catheter. The use of fluoroscopy in locating catheters is somewhat limited, however, in that the physician is only able to visualize the catheter and surrounding tissues in two dimensions. In addition, fluoroscopy does not image soft tissues, making it difficult for the physician to visualize features of the anatomy as a reference for the navigation. Thus, fluoroscopy is sub-optimal for the purpose of navigating a catheter relative to anatomical structure composed primarily of soft tissues, e.g., within the heart.
Various types of three-dimensional medical systems (e.g., the Realtime Position Management™ (RPM) tracking system, developed commercially by Boston Scientific Corporation and described in U.S. Pat. No. 6,216,027 and U.S. patent application Ser. No. 09/128,304, entitled “A Dynamically Alterable Three-Dimensional Graphical Model of a Body Region,” and the CARTO EP Medical system, developed commercially by Biosense Webster and described in U.S. Pat. No. 5,391,199) have been developed, or at least conceived, to address this issue. In these medical systems, a graphical representation of the catheter or a portion thereof is displayed in a three-dimensional computer-generated representation of a body tissue, e.g., a heart chamber. The three-dimensional representation of the body tissue is produced by mapping the geometry of the inner surface of the body tissue in a three-dimensional coordinate system, e.g., by moving a mapping device to multiple points on the body tissue. The position of the device to be guided within the body tissue is determined by placing one or more location elements on the device and tracking the position of these elements within the three-dimensional coordinate system.
These medical systems can also be used to define orifices and vessels by placing the mapping device adjacent to these anatomical locations. In particular, a user may define an orifice, which may represent an ostium of a vessel or a valve opening, by placing the tip of the mapping device at multiple locations on the annular surface around the orifice. However, this can be a tedious and time consuming process, since the tip of the mapping device must be carefully placed on the annular surface that defines the orifice. For example, if the tip of the mapping device is actually placed within the orifice or on a portion of the endocardial surface too far away from the orifice, the resulting graphical image of the orifice will be inaccurate and not very well-defined. A user may also define a vessel by placing the tip of the mapping device through the ostium of the vessel and touching the endothelial surface at various points along the vessel. However, because many points are required to provide an accurate graphical representation of the vessel, and because the tip of a mapping device is not well suited for defining the wall of an elongated anatomical structure, graphical vessel construction using current technologies can also be a tedious and time-consuming process.
There thus remains a need for an improved system and method for graphically defining and representing anatomical orifices and vessels.